Certain known wind turbine blades comprise a root connecting the blade to the hub; a supporting frame connected to the root; and a shell defining the blade section and fitted to the frame.
Wind turbine blades can span considerable lengths, and are subjected to severe stress by the wind, which is transmitted from the shell to the frame, and which the frame is specially designed to withstand.
As shown in U.S. Published Patent Application No. 2009/0136355 A1 and U.S. Published Patent Application No. 2010/0068065 A1, the supporting frame substantially comprises a tubular, substantially rectangular-section spar comprising two opposite parallel caps connected to the shell, and two opposite parallel webs, and which may vary in cross section from the root to the free end of the blade. The caps are positioned directly contacting the shell, may sometimes even form part of the shell and the blade section, and are subjected to bending stress; whereas the webs are subjected mainly to shear stress.
The tubular spar and the shell are made of extremely strong, lightweight plastic reinforced with glass fibers (GFRP), carbon fibers (CFRP), or fibers of other suitable material. And known fabricating methods, as described for example in PCT Patent Application No. WO 2009/153341, PCT Patent Application No. WO 2009/153342 and PCT Patent Application No. WO 2009/153343, comprise molding and cross-linking the two caps and webs; and mainly gluing the caps to the webs to form a tubular spar.
An alternative method is to mold and cross-link two U-shaped members, and glue them together, as described in U.S. Published Patent Application No. 2005/0214122. This solution has the drawback of producing webs with a break along the centreline, and of not allowing use, along the webs, of fibers parallel to the tubular spar axis, thus reducing the structural strength of the webs.
Another tubular spar fabricating method, described in PCT Patent Application No. WO 2010/037762, comprises molding two non-cross-linked L-shaped members, in which three preformed members, some made of non-cross-linked polymers, are embedded in a fiber-reinforced polymer matrix; and gluing the two non-cross-linked L-shaped members together to form a tubular spar. This method involves using two adjacent molds, and simultaneously cross-linking the non-cross-linked preformed members embedded in the L-shaped members, the L-shaped structures themselves, and the glue between the L-shaped structures.
Cross-linking in two adjacent molds forming a closed chamber is a complicated job, and, because of the heat liberated, simultaneously cross-linking the polymer matrix and glue calls for complex, high-cost molds, and increases the risk of rejects. The cost of the method is further increased by the preformed non-cross-linked reinforced-plastic members, which are expensive and involve complex handling procedures.